The invention relates to an optical conductor comprising a fiber, in particular a glass fiber, surrounded by a protective polymer material.
Optical fiber cables are being used more and more commonly for signal transmission, in particular for telecommunications, because of the large passband and the low sensitivity to noise of optical transmission.
Optical conductors are often disposed in a hostile environment. In particular, optical conductors are subjected to moisture and to large temperature variations.
It has been observed that the polymer resins of optical conductors subjected to a hostile environment, in particular a moist environment, are subject to degradation which makes the polymers brittle, leading to the glass fiber becoming stripped and even, in extreme cases, to the conductor breaking.
These defects come from hydrolysis, which causes the molecules of the polymer to separate under the influence of water and which thus modifies the physical properties of the material.
Until now, in order to limit the consequences of moisture, precautionary measures have been multiplied, for example by surrounding the conductor or the set of conductors with a compound such as grease. Another precaution consists in using this type of conductor only at temperatures of not more than 70xc2x0 C.
Protective measures are expensive and not always effective. In addition, the 70xc2x0 C. limit is constricting.
The invention seeks to remedy those drawbacks.
According to the invention the polymer material(s) surrounding the conductive optical fiber, which material(s) is/are of the UV-cured type, contain(s) a water-absorbing additive, in particular a carbodiimide, a poly-carbodiimide, or a polyurea.
To manufacture an optical conductor or ribbon of the invention, the moisture-absorbing additive is incorporated into the polymer resin before it is cured by ultraviolet radiation.
It has been observed that exposure to ultraviolet radiation for curing purposes does not modify the moisture-absorbing properties of the additives. In particular, a mono- or a poly-carbodiimide or a polyurea compound retains its absorption properties.
The action of absorbing water can take place in various ways. For example, the water can form a polymer structure that spoils the physical properties of the resin very little or not at all. If a polyurea compound is used, the polymer formed may be a polyurethane. In other words, the resin is not decomposed by absorbing water.
Tests have shown that the protection provided by the additive increases with the content thereof in the resin. Best results have been obtained for a content of 5% by weight of additive; nevertheless, for reasons of cost, it is preferable not to exceed the value of 5%. The preferred range is 1% to 5% by weight.
The invention also applies to a ribbon, i.e. to a set of optical conductors assembled together in parallel.
It has also been observed that at least some additives added to the polymer material surrounding the optical fiber have an additional beneficial effect of opposing photolysis or photodegradation, i.e. degradation of the polymer during prolonged exposure to light. Usually, such photodegradation gives rise firstly to the polymer resin yellowing, and secondly to the physical properties of the resin being spoilt. A carbodiimide type additive is effective in opposing photodegradation. Best results have been obtained when the carbodiimide content is of the order of 1.5% by weight.
Thus, the invention also relates to an optical conductor comprising a fiber, in particular a glass fiber, surrounded by a protective polymer material of the UV-curable type, and wherein the protective polymer material contains a carbodiimide-based additive that preferably opposes photodegradation.